A quality frequency source is often required for applications such as the local oscillator in a radio frequency transceiver, the frequency reference in a phase-locked loop, or the master clock source in a microprocessor or data-acquisition system. A crystal oscillator is an electronic circuit designed to produce such a stable, accurate frequency signal. In a crystal oscillator, the piezo-electric properties of a quartz crystal or a ceramic resonator used as a circuit element primarily determine the operating frequency. Because of their superior accuracy and stability characteristics, crystal oscillators are standard in a wide variety of electronic devices, from digital watches to “super” computers and almost all radio frequency devices.
Conventional crystal oscillators may be implemented using several different designs. FIG. 1A depicts a “digital” implementation of a conventional Pierce crystal oscillator 10. Crystal 12 is operated in a parallel resonant mode and is very inductive at its resonant frequency, which corresponds to characteristics of the crystal 12. Feedback amplifier 15 is frequently a digital inverter gate. Various implementations of this basic design are quite popular in integrated circuit digital electronics, for example as a clock source for a microprocessor.
Unfortunately, this class of “digital” oscillator produces a very square output with high harmonic content, often swinging from voltage rail to ground rail. Although generally acceptable for digital electronics, such harmonic components are generally undesirable for “analog” applications, e.g., phase locked loops and radio circuitry. In addition, the output of such “digital” oscillators contains large amounts of digital noise. This oscillator has no supply rejection or common mode rejection. Further, when implemented as part of an integrated circuit, due to its single ended or unbalanced nature, it produces deleterious electrical noise, which can be coupled via the IC substrate to other circuits on the integrated circuit, degrading their function.
FIG. 1B depicts an implementation of a conventional Colpitts crystal oscillator 20. Colpitts oscillators are widely used in low noise applications such as phase locked loops (PLLs) and radio circuitry. Transconductance amplifier 25 provides a non-inverting feedback amplifier which causes the circuit to oscillate. The Colpitts topology for a crystal oscillator is generally well regarded in discrete electronic implementations. However, when implemented as part of an integrated circuit, due to its single ended or unbalanced nature, it produces deleterious electrical noise which can be coupled via the IC substrate to other circuits on the integrated circuit, degrading their function.
There are numerous advantages to implementing portions of a crystal oscillator in an integrated circuit. In general, a crystal oscillator implemented in an IC (typically the crystal itself is not part of an IC) is less expensive in terms of component cost and manufacturing expense, and consumes less circuit board area than an implementation based on discrete components. In addition, by implementing the design and most components of the design of a crystal oscillator, integrated circuit manufacturers remove the oscillator design burden from their customers and ensure the quality and reliability of their oscillator implementation.
Unfortunately, for a substantial class of applications, for example, phase locked loops and radio circuitry, existing crystal oscillator designs are not well suited for implementation in an integrated circuit. Such known crystal oscillator designs typically suffer deleterious noise susceptibility. Input noise may be amplified and/or cause an oscillator to lock on to an undesired frequency. Further, they often generate unacceptable noise components on their output signals and frequently couple additional noise via the integrated circuit substrate.